linux-kernel - Re: [RFC PATCH v19 0/8] mm: security: ro protection for dynamic data

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Message-ID: <242fd8a2-2b80-3aa3-4b11-27f49c021a1d@huawei.com>
Date:   Mon, 19 Mar 2018 20:04:35 +0200
From:   Igor Stoppa <igor.stoppa@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     <keescook@...omium.org>, <david@...morbit.com>,
        <rppt@...ux.vnet.ibm.com>, <mhocko@...nel.org>,
        <labbott@...hat.com>, <linux-security-module@...r.kernel.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC PATCH v19 0/8] mm: security: ro protection for dynamic data

On 14/03/18 19:33, Matthew Wilcox wrote:
> I think an implementation of
> pmalloc which used a page_frag-style allocator would be larger than
> 100 lines, but I don't think it would have to be significantly larger
> than that.

I have some doubt about what is the best way to implement it using
vmalloced memory.

1. Since I can allocate an arbitrary number of pages, I think allocating
a rounded up amount of memory, so that it's multiple of PAGE_SIZE should
be enough.

But maybe I could do better than that:
a) support pre-allocation of x pages

b) define, as pool parameter, the minimum number of pages to allocate
every time there is a refill

c) both a and b

----

2. the flavor of page_frag from page_alloc relies on page->_refcount,
however neither vmap_area, nor vm_struct seem to have anything like
that. (My reasoning is that I should do the accounting not on page
level, but based on the virtual area that I get when I allocate new
memory) What would be the best way to do refcounting for the area?

a) use the the page->_refcount from the first page that belongs to the area

b) add the _refcount to either vm_struct or vmap_area (I am not really
sure of why these two structures exist as separate entities, rather than
a single one - cache optimization?)

----

3. I will have to add a list of chunks (in genalloc lingo, or areas, if
we refer to the new implementation), because I will still need to
iterate over all the memory that belongs to a pool, for either write
protecting it or for destroying the pool. I have two options:

a) handle the chunks within the pmalloc pool

b) create an intermediate type of pool (vfrag_pool?) and then include it
in the pmalloc pool structure.

I'd lean toward option a, but I thought I might as well ask for advice
before I implement the less desirable option (whatever it might be).

--
thanks, igor